ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSThere are some unburned or partially burned hydrocarbons in the exhaust.The amount is insignificant from an energy standpoint, but it is objectionable from the viewpoint of itsodour, its photochemical smog, and from the standpoint of its having a carcinogenic effect.The products of photochemical smog cause watering and burning of the eyes, and affect therespiratory system, especially when the respiratory system is marginal for other reasons.HYDROCARBON EMISSIONS FROM SI ENGINESThe most widely accepted causes for hydrocarbon emissions in exhaust gases of spark ignition enginesare:1. Flame quenching at the combustion chamber walls, leaving a layer of unburned fuel-air mixtureadjacent to the walls.2. Crevices in the combustion chamber, small volumes with narrow entrances, which are filled withthe unburned mixture during compression, and remains unburned after flame passages, since the flamecannot propagate into the crevices. The main crevice regions are the spaces between the piston, thepiston rings and the cylinder walls. The other crevice regions are the threads around the spark plug,the space around the plug centre electrode, crevices around the intake and exhaust valve heads, andthe head gasket crevice.3. The oil film and deposits on the cylinder walls absorb fuel during intake and compression, and thefuel vapour is desorbed into the cylinder during expansion and exhaust.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTS4. Incomplete combustion, either partial burning or complete misfire, occurring when the combustionquality is poor, e.g. during engine transients when air-fuel, exhaust gas recirculation, and spark timingmay not be adequately controlled.•All these processes, except misfire, result in unburned hydrocarbons close to the combustion chamberwalls. Mixing of unburned hydrocarbons with the bulk cylinder gases occurs during expansion and theexhaust blowdown processes. During the blowdown process a high concentration of hydrocarbons isreleased from the cylinder through the exhaust valve.•During the exhaust stroke the piston pushes most of the remaining fraction of the cylinder mass withits high hydrocarbon concentration into the exhaust.•The residual gases in the cylinder thus contain a high concentration of hydrocarbons.•Unburned hydrocarbons are thus exhausted in two pulses, the first peak is obtained with the exhaustblowdown and the second occurs towards the end of the exhaust stroke.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSHydrocarbon emissions from CI enginesThe CI engines operate with an overall fuel-lean equivalence ratio, therefore they emit only about one-fifth of the hydrocarbon emissions of an SI engine. The following are the major causes for hydrocarbonemissions in the exhaust of CI engines:1.The diesel fuel contains components of higher molecular weights on average than those in a gasolinefuel, resulting in higher boiling and condensing temperatures.This causes some hydrocarbon particles to condense on the surface of the solid carbon sootgenerated during combustion.Most of this is burned as mixing continues and the combustion process proceeds but a smallamount is exhausted out of the cylinder.2. The air-fuel mixture in a CI engine is heterogeneous with fuel still being added during combustion.It causes local spots to range from very rich to very lean and many flame fronts exist at the same timeunlike the homogeneous air-fuel mixture of an SI engine that essentially has one flame front.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSHydrocarbon emissions from CI enginesIncomplete combustion may be caused by undermixing or overmixing.With undermixing, in fuel-rich zones some fuel particles do not find enough oxygen to react with,and in the fuel-lean zones some local spots will be too lean for combustion to take place properly.With overmixing, some fuel particles may be mixed with burned gases and it will therefore leadto incomplete combustion.3. A small amount of liquid fuel is often trapped on the tip of the injector nozzle even wheninjection stops. This small volume of fuel is called sac volume.This sac volume of liquid fuel is surrounded by a fuel-rich environment and therefore itevaporates very slowly causing hydrocarbon emissions in the exhaust.4. CI engines also have hydrocarbon emissions for some of the same reasons as SI engines do,e.g. flame quenching, crevice volume, oil-film and deposits on the cylinder wall, misfiring, etc.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSCarbon Monoxide (CO)Carbon monoxide is toxic. The haemoglobin in the blood, which carries oxygen to the different partsof the body, has a higher affinity for carbon monoxide than for oxygen. The percent carboxyhaemoglobin gradually increases with time to an equilibrium value which depends upon the carbonmonoxide concentrations.Carbon monoxide is generated in an engine when it is operated with a fuel-rich equivalence ratio asthere is not enough oxygen to convert all carbon to carbon dioxide. For fuel-rich mixtures, COconcentrations in the exhaust increase steadily with the increasing equivalence ratio. The engineruns rich when it is started or when it is accelerated under load. For fuel-lean mixtures, COconcentrations in the exhaust are very low and are of the order 10-3mole fraction.Poor mixing, local rich regions, and incomplete combustion create some CO. The SI engines oftenoperate close to stoichiometric at part load and operate fuel rich at full load. Under these conditions,the CO emissions are significant. However, CI engines operate well on the lean side ofstoichiometric and therefore produce very little CO emissions.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSOxides of Nitrogen (NOx)• The oxides of nitrogen tend to settle on the haemoglobin in the blood. The most undesirable toxiceffect of oxides of nitrogen is their tendency to join with the moisture in the lungs to form dilutenitric acid. NOx is one of the primary causes of photochemical smog (smoke + fog). Smog is formedby the photochemical reaction as follows:NO2 + energy from sunlight NO + 0 + SmogMonoatomic oxygen reacts with O2 to form ozone (03) as follows:O + O2 O3• Ozone is harmful to lungs and other biological tissues. It is harmful to crops and trees. It reactswith rubber, plastics and other materials causing damage.• Most of the oxides of nitrogen comprise nitric oxide (NO), a small amount of nitrogen dioxide(NO2) and traces of other nitrogen oxides. These are all grouped together and the group is calledNOx.• NOx is mostly formed from atmospheric nitrogen. There are a number of possible reactions thatform NO. NO forms in both the flame front and the post flame gases.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSSome of the NO forming reactions are:• N, O, OH are formed from the dissociation of N2, O2 and H2O vapour at high temperatures thatexist in the combustion chamber (2500-3000 K).• The higher the combustion reaction temperature, the more diatomic nitrogen (N2) will dissociateto monatomic nitrogen (N) and more NOx will be formed.• At low temperatures, a very small quantity of NOx is created. The flame temperature ismaximum at the stoichiometric equivalence ratio (0 = 1.0) but maximum NOx, is formed slightlyat a lean equivalence ratio (0 = 0.95). At this condition the flame temperature remains very highbut excess oxygen helps in the formation of more NOx. The most important engine variables thataffect NOx emission are the fuel/air equivalence ratio, the burned gas fraction (EGR and residualgas fractions) and combustion duration within the cylinder. NOx is reduced in modem engineswith fast-bum combustion chambers.• If ignition spark is advanced, the cylinder temperature will be increased and more NOx will beproduced. CI engines with divided combustion chambers and indirect injection (DI) tend togenerate higher levels of NOx.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSPARTICULATES• The particulates from SI engines are lead, organic particulates including soot and sulphates.Gasoline may contain some sulphur, which is oxidized within the engine cylinder to form SO2.• is oxidized to SO3 which combines with water to form a sulphuric acid aerosol.• Leaded gasolines emit lead compounds. Soot emissions (black smoke) can result fromcombustion of overly rich mixtures. In properly adjusted spark-ignition engines, soot in theexhaust is not a significant problem.• Diesel particulates consist mainly of combustion generated carbonaceous material (soot) onwhich some organic compounds have been absorbed.• Most particulates are generated in the fuel rich zones within the cylinder during combustion dueto incomplete combustion of fuel hydrocarbons; some particulate matter is contributed by thelubricating oil.• These are undesirable odorous pollutants. Maximum particulate emissions occur when the engineis under load. At this condition, maximum amount of fuel is injected to obtain maximum powerfrom the engine. It results in a rich mixture and poor fuel economy.• At temperatures above 500°C, soot particulates appear as clusters of a large number of solidcarbon spheres with individual diameters of about 15 to 30 nm.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFORMATION OF POLLUTANTSPARTICULATES• As the temperature decreases below 500°C during expulsion, the particles become coated with HCand with traces of other components.• The words particulates and soot are often used synonymously, but there is a difference in naturebetween these two emissions.• Dry soot is usually the carbon that is collected on a filter paper in the exhaust of an engine.• The unit of measurement of soot is usually the Bosch Smoke Number, which is assessed by thereflectance of a filter paper on which the soot has been collected.• Particulates contain more than simply the dry soot; they are the soot particles on which the othercompounds, often the polycyclic aromatic hydrocarbons (PAH), have condensed.• The PAH compounds have a tendency to be carcinogenic.• The level of particulates increases with the sulphur content in the fuel. Particulates are measured bytrapping the particles on glass-fibre filter papers placed in a dilution tunnel, and then weighing thequantity.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringMEASUREMENT OF POLLUTANTS• The measurement of exhaust emissions is very important for the control of air pollution from ICengines.• CO concentrations are measured by infrared absorption,• NO concentrations are measured by chemi-luminescence and• Unburned HC are measured by flame ionization detector.Non-dispersive Infra-red (NDIR) Analyzer• The NDIR analyzers are used for measuring the concentrations of carbon monoxide and carbondioxide. This device is based on the principle that the infrared energy of a particular wavelength,peculiar to a certain gas, will be absorbed by that gas. The infrared energy of other wavelengths willbe transmitted by that gas.• Carbon dioxide absorbs infrared energy in the wavelength band of 4 to 4.5 microns (m) andtransmits the energy of the surrounding wavelengths. The carbon monoxide absorption band isbetween 4.5 and 5 microns (gm).
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• Nitric oxide (NO) has also a weak absorption band,allowing it to be analyzed by NDIR, but lack ofsensitivity and interference by water vapour do not givehigh accuracy with low concentrations.• A schematic arrangement of the IR analyzer is shown inFigure.• A wideband infrared radiation source consists of a heatedwire, which is placed in a quartz tube mounted in thesource block.• Radiation from the source is reflected within themounting block and passes out of a symmetrical pair ofrectangular apertures as two parallel beams into the twoseparate cells a sample cell and a reference cell.• These cells are internally highly polished and gold platedto ensure high transmission of radiation.• After passing through these cells the infrared radiation isreceived in two separate detector cells, which are full ofthe gas whose concentration is to be measured.Non-dispersive Infra-red (NDIR) Analyzer
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• The two detector cells contain equal amounts of this gasand are separated by a flexible diaphragm.• The sample cell is a flow-through tube that receives acontinuous stream of the mixture of gases to be analyzed.• When the particular gas to be measured is present in thesample, it absorbs the infrared radiation at its characteristicwavelengths. The percent of radiation absorbed isproportional to the molecular concentration of thecomponent of interest in the sample.• The sample cells may be divided by quartz windows intovarious lengths to give different ranges of sensitivity.• The quartz windows do not absorb infrared energy in theregion of interest. Low concentrations are best measured bylonger cells so that more molecules of interest are present.• The unused sample cells are generally flushed with a non-infrared absorbing gas such as oxygen or nitrogen, or with agas free of the components being measured, e.g. fresh airfor carbon monoxide analyzers.Non-dispersive Infra-red (NDIR) Analyzer
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• The reference cell is sealed and is physically identical to thesample cell. It is filled with an inert gas (usually nitrogen)which does not absorb the infrared energy of the characteristicwavelength of the species of interest.• The radiant energy, after passing through the cells, heats thegas in the corresponding chamber of the detector. Since noradiant energy is absorbed in the reference cell, thecorresponding chamber in the detector is heated more and itspressure becomes higher than that in the other chamber.• This pressure differential causes the diaphragm to move andvary the capacitance. Therefore, the variation in thecapacitance is proportional to the concentrations of the speciesof interest in the exhaust sample.• The radiation from the source is interrupted by a rotating two-bladed shutter driven by a synchronous motor.• The shutter is placed between the infrared source and the cells.Non-dispersive Infra-red (NDIR) Analyzer
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• When the shutter blocks the radiation, the pressure in the twocompartments of the detector is equal because there is noenergy entering either of the chambers of the detector.• This allows the diaphragm to return to its neutral position. Asthe shutter alternatively blocks and unblocks the radiation,the diaphragm fluctuates causing the capacitance to chargecyclically.• This sets up an ac signal, which is impressed on a carrierwave provided by a radio-frequency oscillator (amplificationsof ac signals have better drift-free characteristics than theamplifications of dc signals). Additional electronic circuitryin the oscillator unit demodulates and filters the resultantsignal.• This signal is then amplified and rectified to a de signalwhich is measured by a meter or recorder. The final dc signalis a function of the concentration of the species of interest inthe exhaust sample.Non-dispersive Infra-red (NDIR) Analyzer
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• To set the zero point, a non-infrared-absorbing gas, e.g.dry air, is passed through the instrument. For the otherpoints on the scale, calibrating gases with knownconcentrations are passed through the analyzer.• An error in the NDIR readings may arise if the exhaustsample contains other species that can absorb radiation atthe same frequencies that the gas in the detector willabsorb.• In order to minimize this interference, a largeconcentration of the interfering gas is placed in the filtercells.• The analyzer zero is then set with this large concentrationof the interfering gas.Non-dispersive Infra-red (NDIR) Analyzer
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFLAME-IONIZATION DETECTOR (FID)• Some hydrocarbons have an infrared absorption at 3.4microns, but some others, notably aromatics, have almostnone. Only about 50 % of exhaust hydrocarbons ismeasured by NDIR, therefore, this method is not suitablefor the measurement of HC concentrations.• The flame ionization detector is mainly used to measurethe unburned hydrocarbon concentrations in the exhaustgases. It is based on the principle that pure hydrogen-airflames produce very little ionization, but if a fewhydrocarbon molecules are introduced the flames producea large amount of ionization. The ionization is proportionalto the number of carbon atoms present in the hydrocarbonmolecules.• A schematic arrangement of the instrument is shown inFigure. It consists of a burner assembly, an ignitor, an ioncollector and electric circuitry. The burner consists of acentral capillary tube.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringFLAME-IONIZATION DETECTOR (FID)• Hydrogen, or a mixture of hydrogen and nitrogen,enters one leg of the capillary tube and the sampleenters through another leg. The length and bore of thecapillary tubes are selected to control the flow rates.The mixture of H2 - N2 - C„H,„ then flows up theburner tube.• The air required for combustion is introduced fromaround the capillary tube.• The combustible mixture formed in the mixingchamber is ignited by a hot wire at the top of theburner assembly and a diffusion flame stands at theexit to the burner tube.• An electrostatic field is produced in the vicinity of theflame by an electric polarizing battery.• This causes the electrons to go to the burner jet andthe positive ions go to the collector.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• The collector and the capillary tube form part of anelectric circuit.• The flow of ions to the collector and the flow ofelectrons to the burner complete the electrical circuit.• The dc signal produced is proportional to the number ofions formed and the number of ions is proportional to thenumber of carbon atoms in the flame.• The dc signal generated is attenuated by a modulator andthen fed to an ac amplifier and a demodulator.• The signal is then recorded on a meter. The meter iscalibrated directly in amount of hydrocarbonconcentrations.• To calibrate, the samples of known concentration ofhydrocarbons are fed to the instrument and the meterreadings are adjusted accordingly.FLAME-IONIZATION DETECTOR (FID)
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringCHEMILUMINESCENCE ANALYZERS (CLA)• The chemilutilinescent analyzer measures the nitric oxide (NO) concentrations. This technique isbased on the principle that NO reacts with ozone (03) to give some NO2 in an electronicallyexcited state. These excited molecules on decaying to the ground state emit red light (photons) inthe wavelength region from 0.6 gm to 3 gm, i.e.• NO + 03 NO2* + 02• NO2* --> NO2 + hv• where h is Plancks constant and v represents a photon of light.• The oxides of nitrogen (NO„) from the engine exhaust comprise mainly a combination of nitricoxide (NO) and nitrous oxide (NO2).• By converting any exhaust NO2 to NO in a thermo-catalytic converter before supplying theexhaust gas to the analyzer, the value of total nitrogen oxides (N0x) can be obtained.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringCHEMILUMINESCENCE ANALYZERS (CLA)
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• A schematic arrangement of the chemiluminescent instrument is shown in Figure. The vacuum pumpcontrols the pressure in the reaction chamber and draws ozone and the exhaust sample. The ozone isproduced by an electric discharge in oxygen at low pressure.• An NO2-to- NO converter is also shown in the diagram. An arrangement is made by using a bypass line,so that it may be possible to measure only the NO concentrations or NO + NO2, i.e. NOx concentrationsin the combustion engine exhaust.• A mixture of a gas sample and ozone enters a reaction chamber (reactor) which is maintained at a verylow absolute pressure. The reaction of the ozone and nitric oxide when heated under vacuum at 600°Cproduces some electronically excited molecules of NO2.• The electronically excited molecules on decaying, emit light. The light can readily be detected accuratelyby a photo- multiplier.• The signal is then amplified and fed to a recorder. Many parameters affect light emission in the reactor,it is therefore essential to calibrate the analyzer regularly.• Pure nitrogen may be used for zero setting. The zero control is adjusted until the digital voltmeter readszero, the nitrogen gas is then disconnected and a standard NO / N2, mixture is connected.• The NO/NOx, switch is set to NO mode and the span control is used to adjust the NO reading tocorrespond with the standard. For the NOx reading the NO/NOx, function switch is pressed to initiatethe NOx mode.CHEMILUMINESCENCE ANALYZERS (CLA)
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringA dilution tunnel is used to measure the amount of particulate present in the exhaust gas fromthe diesel engine. In the dilution tunnel, the exhaust gases are diluted with ambient air to atemperature of 52°C or less, and a sample stream from the diluted exhaust is filtered to removethe particulate material.The particulate is trapped after dilution because the particulate gets condensed over the filter atthis temperature. The amount of particulate trapped is obtained by weighing the filter beforeand after the experiment.MEASUREMENT OF PARTICULATESMEASUREMENT OF EXHAUST SMOKESmoke-meters are used to measure the intensity of exhaust smoke. Smoke-meters may measure eitherthe relative quantity of light that passes through the exhaust gas (Hartridge smoke-meter), or therelative smudge left on a filter paper (Bosch smoke-meter).
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of EngineeringHARTRIDGE SMOKE-METERIt is based on the principle that the intensity of a light beam is reduced by smoke which is a measureof smoke intensity. A schematic diagram to illustrate the principle of this smoke-meter is shown inFigure.Light from a source is passed through a standard length of a tube where the exhaust gas sample iscontinuously supplied from the engine and at the other end of the tube the transmitted light ismeasured by a photo-electric cell.The photoelectric cell converts the light intensity to an electric signal, which is amplified andrecorded on a meter. The intensity of smoke is expressed in terms of smoke density. It is defined asthe ratio of electric output from the photoelectric cell when an exhaust sample is passed through thetube to the electric output when clean air is supplied.
ME2041AdvancedInternal CombustionEnginesUnit II Department of Mechanical Engineering, St. Joseph’s College of Engineering• It is based on the principle that when a certain quantity of exhaust gas passes through a fixed filterpaper, some smoke smudge is obtained on it, which is a measure of smoke intensity.• A schematic diagram to illustrate the principle of this instrument is shown in Figure.• A fixed quantity of the exhaust gas from the engine is introduced into a tube, where it passesthrough a fixed filter paper. Depending upon the smoke density, some quantity of smudge isdeposited on the filter paper, which can be evaluated optically.• A pneumatically-operated sampling pump and a photoelectric unit are used for the measurement ofthe intensity of smoke smudge on the filter paper.BOSCH SMOKE-METER